The present invention relates to a process for electrochemical reduction of vat and sulfur dyes in aqueous solutions and equipment for carrying out the process.
The applications of vat and sulfur dyes on cellulosic materials takes place in the reduced form, since only these are water-soluble and possess a high affinity to the substrate. Through the oxidation carried out after the dyeing, the dye is again converted from its leuco form into the water-soluble pigment structure.
The application of vat and sulfur dyes for printing and dying of cellulosic fibers has up to now been connected to the introduction of over-stoichiometric reducing-agent quantities (with respect to the quantity of dye to be reduced). The reduction of the vat dyes takes place usually in alkaline (pH greater than 9) aqueous solutions with sodium dithionite (hydrosulfite) or reducing materials derived therefrom (RONGALIT C, BASF) in connection with wetting and complexing agents. Other reducing agents such as thiourea dioxide or endiolate have hardly gained acceptance on the basis of cost, while in the case of thiourea dioxide, an environmental problem exists similar to that with hydrosulfite.
Reducing agents suitable for the reduction of vat dyes under the conditions necessary for vatting of dyes exhibit an oxidation-reduction potential of xe2x88x92400 mV to xe2x88x921,000 mV. Both the use of hydrosulfite as well as thiourea dioxide lead to a high sulfite or sulfate pollution of the waste water. These salt loads, on the one hand, are toxic and, on the other hand, are corrosive and lead to destruction of the concrete ducts. Another problem caused by the sulfate load arising from the sulfite in the waste water is the formation by anaerobic organisms of hydrogen sulfide in the drain pipes.
Even newer processes can only partially solve the mentioned problem; here worth mentioning is the reduction in an ultrasonic reactor in connection with the usual reducing agents or the electrochemical vatting with the help of a mediator. The vatting in an ultrasound reactor offers the advantage that the reducing agent usage can be reduced to the stoichiometric ratio and the hydrosulfite can be replaced with endioles.
The direct electrochemical reduction of dye pigments has not been realized up to now. A known electrochemical process utilizes hydrosulfite, from which other reaction products arise that reduce the dye, which products lead to a diminishing of the quantity of the application of hydrosulfite necessary for dye vatting (E. H. Durawalla, Textile Asia, 165-9, September 1975).
Another known process utilizes oxidation-reduction systems such as, for example, iron (II)- or iron (III)-complexes (T. Bechtold et al., Angew. Chem. Int. Ed. English 1992, 31, No. 8, 1068-9; WO 90/15182).
With all these electrochemical vatting processes the agent that reduces the dye is the applied reducing agent or mediator. The mediator system is electrochemically cathodically regenerated according to the example mentioned above (e.g., Fe2+ less than xe2x88x92 greater than Fe3+). Due to the high usage quantity and the disturbing ecological nature of such mediators, an acute environmental problem arises after as well as before, which can be resolved only with additional investment in appropriate waste water technology or through a recycling process. Another disadvantage of the process is the permanent replenishing of the mediator for maintaining the oxidation-reduction cycle in continuous dyeing technology. The replenishing of the mediator system arises from the bath discharge that is proportional to the fabric or thread flow.
Up to now it was not possible to reduce vat dyes electrochemically on a commercial scale without the addition of a mediator. The causes of the mentioned difficulty are predetermined by the dye pigment, since this exhibits a completely inert behavior in an electrolysis cell, through its lack of solubility in water.
The object of the present invention is therefore, while avoiding the mentioned disadvantages of known reducing processes, to make available a vat dyeing process generally free of reducing agents for the production of completely reduced dye solutions for the dyeing of cellulosic textile materials.
The object is solved through a process for electrochemical reduction of vat and sulfur dyes in aqueous solutions, characterized by the fact that two dye-radical anions (2R) are formed (reaction equation I) in a com-proportionization reaction between a dye (A) and its reduced form (P), resp. species (P), that the two dye-radical anions (2R) are reduced electrochemically (reaction equation (II)) to the same species,
A+Pxe2x86x922Rxe2x80x83xe2x80x83(I)
2R+2exe2x88x92xe2x86x922Pxe2x80x83xe2x80x83(II)
that the reaction equations (I) and (II) form a steady-state cycle, that the reaching of the steady-state reaction conditions is effected through a start reaction, and that the steady-state cycle is maintained, whereby the formed species (P) on the one hand is necessary for maintaining the circuit and on the other hand is used for the dyeing process. The object is also solved through equipment for carrying out the process according to one of the claims 1-10, characterized by the fact that for a dye suspension (A) located in an electrolysis vessel (1) provision is made for a circuit with a circulation stream (V1xe2x80x2), whereby the electrolysis vessel (1) is fitted out with electrodes 6, 6xe2x80x2, that provision is made for a like dye suspension, located in a second vessel (11), for introduction with a first volume stream (V2xe2x80x2) into the circuit via conduits (14, 14xe2x80x2) and a pump (P2), that the electrolysis vessel (1) is fitted out with second conduits (15, 15xe2x80x2) and a second pump (P3) for the removal of a volume stream (V3xe2x80x2) of a quantity equivalent to the first volume stream (V2xe2x80x2), whereby the second conduit (15xe2x80x2) is connected to a third vessel (21).